1. Field of Invention
The present invention relates to electrical protective filters or transient voltage surge suppressors (TVSS) for office equipment or any microprocessor-based equipment. These suppressors provide surge filtration to protect the office equipment from surges or transients in the power furnished from the power distribution network.
2. Description of the Prior Art
Numerous types of office equipment used in recent years contain electronic chips or components which are sensitive to voltage surges or transients in the power distribution network provided by utility companies. Examples of such types of office equipment include computers, facsimile machines, photocopiers and the like. Devices known as power filters or transient voltage surge suppressors have been developed to protect these types of office equipment from such power surges or transients. U.S. Pat. No. 5,721,661, of which applicant is inventor, is one form of such a power filter.
Office buildings and other places where such equipment has been used have often been based on a three-phase or center-tap system of power distribution. There are several advantages from the use of this form of power. It allows more power to be delivered to a site, is more easily distributed, and allows two voltage levels (such as 120 volts and 240 volts) to be delivered. Customers can thus use equipment requiring larger amounts of power. These forms of power are a more cost effective way for an electric utility to transmit power to an end user. In three phase or center tap power, in the event of loss of the ground wire or neutral wire connection in the distribution grid, voltage levels provided in the network could increase up to a double level. This is not an uncommon event. Equipment rated at 120 volts could thus be subjected to up to 240 volts. So it could possibly burn out a surge protection component and possibly cause firing the TVSS unit, which could also burn out the connected equipment.
So far as is known, it was typical to use relatively inexpensive thermal fuses for protective purposes in these situations. These fuses would typically open after response to excess heat for periods of from couple of seconds to several minutes. It may protect the TVSS against firing after burn out, but sensitive electronic chips and circuits of connected equipment were not capable of withstanding such excessive voltage levels for even short fractions of a second, such as a few milliseconds.
Briefly, the present invention provides a new and improved protective circuit for electrical apparatus. The protective circuit has hot, neutral and ground leads which are arranged to be placed between corresponding hot, neutral and ground leads of the electrical circuit. The protective circuit includes a voltage surge protection and filtration circuit connected between the hot and neutral leads, also between neutral and ground, and also between the hot lead and ground.
The voltage surge protection circuit has first and second voltage surge protectors connected in an electrical series relationship with each other. The first and second voltage surge protectors are connected at a common connection or node to each other. The first and second voltage surge protectors are connected between the neutral lead and ground lead to suppress and filter neutral ground noise and transients. If desired, a second set of first and second voltage surge protectors, similarly connected to each other and connected to the first set through a noise isolation transformer or differential transformer may be included.
The protective circuit also includes a series-connected set of first and second relays connected between the hot and neutral, as well as hot and ground, leads of the protective circuit. The first relay has a switch arm to connect the clamping device, such as an MOV gas tube, Zener or any other type of voltage clamping component, also to connect a filtration component such as capacitor or inductor between the neutral lead and ground lead to suppress and filter the neutral ground (common mode) noises and spikes when the first relay is receiving current. The first relay is typically receiving current when the ground lead is properly connected to ground, also the hot and neutral leads are correctly connected to hot and neutral, not reverse polarity.
The first relay switch arm opens when current in the first relay circuit is not being received or present, such as when the ground connection of the ground lead is interrupted or broken. Opening the first relay switch arm isolates the electrical apparatus from the power utility. It also protects the voltage surge protectors and filtration components. The protective circuit of the present invention thus protects the voltage surge protectors and filtration components such as MOV""s and capacitors and any type of voltage protector devices (such as a gas-tube, ZENER or the like) between neutral and ground or hot to ground in the voltage surge protector circuit. Protection is provided against over voltage (240 V) when connection to an electrical ground is broken or reverse polarity of hot and neutral causing voltage increase up to 240 V.
The second relay of the protective circuit also controls a switch arm which is located in a conductor connecting the hot lead of the power utility outlet to the hot lead of the electrical apparatus when the second relay is receiving current. The second relay switch arm opens when current in the second relay is not present, such as when the ground connection of the ground lead is interrupted or a reverse polarity of hot and neutral or when the voltage increases up to 240 V.
The protective circuit of the present invention also protects all components between hot and neutral and thus protects the electrical apparatus when ground connection to electrical ground is broken or interrupted. Protection occurs against reverse polarity of hot and neutral or when voltage increases up to 240 V, and also in the event of applying 240 volts to hot and neutral without ground connection.
The protective circuit of the present invention also includes a voltage threshold sensing circuit which detects when the voltage between the hot and neutral lead of the power utility outlet exceeds an established protective level. An electronic switch is connected to the voltage threshold sensing circuit. The electronic switch responds to power from the utility outlet detected as exceeding the threshold and disables the second relay which supplies current to the connected apparatus or microprocessor-based equipment. The electrical apparatus is thus protected by the circuit of the present invention from excess voltage levels between the hot and neutral lead from the power utility outlet. An indicator, such as an LED, also turns on to indicate the abnormality of voltage.